1. Field of the Invention
The present invention relates to an active matrix liquid crystal display.
2. Description of the Related Art
Active Matrix Liquid Crystal Displays (AMLCDs) may be fabricated with polysilicon thin film transistors (TFTs) such that display driver circuitry is integrated onto the same substrate as the active matrix. Such TFTs may exhibit threshold voltages significantly greater than those exhibited by transistors fabricated in a standard CMOS IC (integrated circuit) process. Consequently, the TFT circuitry requires a significantly higher supply voltage than CMOS ICs. In addition, the switching voltage of the liquid crystal material may be significantly larger than the supply voltage of a CMOS IC.
As shown in FIG. 1 of the accompanying drawings, a known AMLCD module 1 comprises: a display substrate 2 with an active matrix 3 of pixels (picture elements); source and gate line driver circuits 4, 5 integrated on the display substrate 2; a liquid crystal display controller (LCDC) IC 6; a flexible printed circuit (FPC) 7 on which the LCDC 6 is mounted; and a backlight 8. In order to minimise power consumption of the display module 1, it is desirable to supply the module with as low a power supply voltage as possible. This will typically be the supply voltage required by the liquid crystal display controller 6. The display module 1 must then also contain a DC-DC converter 9 as a means of boosting the supply voltage to the level required by the integrated TFT circuitry. The DC-DC converter 9 is typically included in the module in the form of an IC chip and may be found, along with the LCDC 6, on the FPC 7.
In order to reduce the number of components, and hence the cost, of the display module 1, it is desirable to integrate the DC-DC converter 9 onto the display substrate 2, as shown in FIG. 2 of the accompanying drawings. A problem with integrating the DC-DC converter 9 onto the display substrate 2 is that the TFT circuits constituting such an integrated converter do not operate unless a supply voltage significantly higher than that supplied to the LCDC 6 is provided.
A known DC-DC converter that has been suggested as suitable for integration onto the display substrate is disclosed in Y. Nonaka et. al. “A DC-DC Converter Circuit Integrated into a Poly-Si TFT-LCD Containing a 6-bit DAC”. Society for Information Display, Proceedings, 2003. Although this circuit requires a smaller supply and input signal voltage than other TFT circuitry integrated onto the display substrate, it relies on the supply voltage being larger than the threshold voltage of the constituent transistors. Consequently, this circuit does not allow power consumption to be minimised since the display module must be supplied with a voltage larger than that required by the LCDC alone.
A voltage-boosting circuit suitable for integration onto the display substrate that overcomes the problem of high TFT threshold voltages is disclosed in U.S. Pat. No. 6,404,271. This publication describes a version of the well-known Dickson charge-pump circuit which is disclosed in J. Dickson. “On-chip high-voltage generation in MNOS integrated circuits using an improved voltage multiplier technique”. IEEE Journal of Solid State Circuits, June 1976 in which the switching elements are formed by gated PIN diodes. The PIN diodes allow the use of a lower power supply voltage. However, a problem with this circuit is that it is inefficient. The PIN diodes present a significant on resistance and capacitance and a significant leakage current. This acts to reduce the output voltage and drive capability of the circuit.